1. Technical Field
The present invention relates to an acoustic resonator and a filter, and particularly to an acoustic resonator, which is capable of suppressing a spurious occurrence, and a filter using the acoustic resonator.
2. Background Art
Components embedded in electronic device such as portable devices are required to decrease in size and weight. For example, a filter used in portable devices is required to decrease in size, and also required to be capable of precisely adjusting a frequency characteristic. A filter using an acoustic resonator is known as a filter satisfying such requirements (refer to Patent Documents 1 to 3).
Hereinafter, a conventional acoustic resonator disclosed in Patent Document 1 will be described with reference to FIGS. 13A to 13D.
FIG. 13A is a cross-sectional view showing a fundamental structure of the conventional acoustic resonator. The conventional acoustic resonator has a structure in which a piezoelectric body 101 is provided between an upper electrode 102 and a lower electrode 103. This conventional acoustic resonator is used while mounted on a substrate 105 in which a cavity 104 is formed. The cavity 104 can be formed by performing, on the back of the substrate 105, partial etching using microfabrication method. In this conventional acoustic resonator, when a electric field is applied in a thickness direction via the upper and lower electrodes 102 and 103, a vibration occurs in the thickness direction. Next, operations of the conventional acoustic resonator will be described referring to a thickness longitudinal vibration occurring in an infinite plane.
FIG. 13B is a schematic perspective view for describing the operations of the conventional acoustic resonator. When an electric field is applied between the upper and lower electrodes 102 and 103 in the conventional acoustic resonator, an electrical energy is converted into a mechanical energy at the piezoelectric body 101. A mechanical vibration induced here is an extensional vibration in the thickness direction, which vibration extends and contracts in a same direction as that of the electric field. In general, the conventional acoustic resonator uses a resonant vibration in the thickness direction in the piezoelectric body 101. The conventional acoustic resonator operates to resonate at a frequency whose half wavelength is equivalent to a thickness of the piezoelectric body 101. The cavity 104 shown in FIG. 13A is used for obtaining a thickness longitudinal vibration of the piezoelectric body 101.
As shown in FIG. 13D, in an equivalent circuit of the conventional acoustic resonator, both a serial resonance and a parallel resonance occur. The equivalent circuit comprises: a serial resonance section including a capacitor C1, inductor L1 and resistor R1; and a capacitor C0 connected in parallel with the serial resonance section. As shown in FIG. 13C, an admittance frequency characteristic of the equivalent circuit in this circuit configuration has an admittance which becomes maximum at a resonance frequency fr and minimum at an antiresonance frequency fa. Here, a relationship between the resonance frequency fr and antiresonance frequency fa is as follows:fr=1/{2π√{square root over ( )}(L1×C1)}fa=fr√{square root over ( )}(1+C1/C0)
When the conventional acoustic resonator having the above admittance frequency characteristic is applied as a filter, the resonant vibration of the piezoelectric body 101 is utilized. This allows the filter to be realized as a small-sized low-loss filter.
Here, a piezoelectric thin film which greatly affects a characteristic of the acoustic resonator is desired to be of high quality. For this reason, various manufacturing methods for realizing a high-quality piezoelectric thin film have been suggested (refer to Patent Document 4). FIG. 14 illustrates steps of a conventional acoustic resonator manufacturing method disclosed in Patent Document 4.
First, etching is performed on a substrate 111 to form, on the substrate 111, a hollow which is later formed into a cavity 112 (FIG. 14(a)). Then, a sacrifice layer 115 is formed on an entire surface of the substrate 111 (FIG. 14(b). Next, planarization is performed such that the surface of the substrate 111 and a surface of the sacrifice layer 115 have a same height (FIG. 14(c)). Subsequently, a lower electrode 121, piezoelectric thin film 122 and upper electrode 123 are laminated thereon, whereby a vibration section 120 is formed (FIG. 14(d)). Lastly, the sacrifice layer 115 is removed by etching, whereby the cavity 112 is provided, and thus the acoustic resonator is completed (FIG. 14(e)). In Patent Document 4, a film thickness and flatness of molybdenum (Mo), which is the lower electrode 121, are specified so as to improve crystallinity of the piezoelectric thin film. This allows a piezoelectric thin film having a high quality to be realized by using a spatter method.
Patent Document 1: Japanese National Phase PCT Lain-Open Publication No. 60-68711
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-158309
Patent Document 3: U.S. Pat. No. 5,587,620
Patent Document 4: Japanese Patent No. 2800905
Patent Document 5: U.S. Pat. No. 6,060,818